English is not my native language. When I speak to my friends in English, they understand, but native-English-speakers sometimes don't understand. Are my friends more intelligent?

This is a problem in all languages, of course, not only English. The better you know a language, the more possibilities you know. If you take one word, and change its pronunciation just a little, it may have another meaning. In fact, to a native-speaker one word may have many different meanings, depending on the exact pronunciation and/or context. So if you don't pronounce a word correctly, or you use it in the wrong context, a native speaker has to consider perhaps ten or more different meanings that your sentence may have. To you and your friends it's simple, because you think there is only one meaning. But the native-speaker has to calculate many possible combinations or "permutations", like a computer. That's why he or she may seem a little "stupid" to you!